1. Field of the Invention
The invention relates to a new method of combining GPS and navigation estimates to provide GPS acquisition in the presence of interfering signals.
2. Description of the Related Art
The Global Positioning System has revolutionized navigation. Prior to the twentieth century, land and sea navigation was performed using angle measurements to celestial bodies, magnetic compass measurements, and time measurements from an accurate clock to determine longitude. At the beginning of the twentieth century, the first airplane flight foreshadowed the need for more accurate navigation systems. Radio and inertial navigation systems soon supplanted the old methods.
The GPS concept was developed in the early 1970s as a joint military service project and the first satellite was launched in 1978. GPS satellites transmit time synchronized signals in the L frequency band (approximately 1.5 GHz) using atomic clocks. The signals are BPSK modulated by a spread spectrum ranging code and navigation message. The code, known to the user, is unique for each satellite. The navigation message contains data on satellite ephemeris and clock errors, atmospheric corrections, and general satellite almanac data.
A GPS navigator determines position with time-of-arrival measurements. The navigator correlates a replica of each satellite's code with the received satellite's signal. Accurate knowledge of the code transmit time from each satellite and the receive time at the navigator will determine pseudoranges, ranges which contain an offset because of navigator clock error. Four satellites are the minimum required for determination of a vehicle's three dimensional position and clock offset.
An inertial navigation system, INS, incorporates gyros and accelerometers mounted along three orthogonal axes to measure vehicle angular rates and specific force respectively. An INS measures short term vehicle dynamics and is, therefore, a perfect companion to GPS to achieve high dynamic and high accuracy vehicle navigation.
Standard GPS receivers employ tracking loops to generate pseudorange and delta pseudorange measurements for each satellite. The GPS signal at the vehicle's antenna is amplified, down converted, and digitized prior to entering the tracking loops. It is a composite signal, containing information from all visible satellites.
In each satellite tracking loop, the incoming carrier signal is mixed with a replica of the carrier to produce in-phase (I) and quadrature-phase (Q) signals with a small phase error. These Is and Qs are transformed into an explicit phase error signal, integrated, subsampled, filtered and fed to a numerically controlled oscillator to generate the replica carrier signal. This process is referred to as the carrier tracking loop.
An inner loop correlates the Is and Qs with on time, one half chip early, and one half chip late replica codes for the specific satellite being tracked. The Is and Qs are now only a function of the code correlation and phase error. These are transformed into an explicit pseudorange error signal, integrated, subsampled, filtered, and fed to a numerically controlled oscillator to clock the satellite replica code. This inner loop is referred to as the code tracking loop.
A major tracking loop design tradeoff is the ability to handle high vehicle dynamics and to also track through high interfering noise. One way of solving this problem is to aid the carrier tracking loop with an Inertial Navigation Unit (IMU) and let the carrier loop aid the code loop. The IMU will remove short term dynamics from the tracking loop and thus allow the loop bandwidth to be reduced. Lowering the bandwidth results in greater noise suppression.
The output of each satellite's tracking loop includes pseudorange and delta pseudorange measurements. These, along with satellite ephemeris and IMU measurements, are input to a navigation filter which estimates vehicle position, velocity, attitude, user clock parameters, and IMU error terms. In real-time systems, the navigation filter usually runs at a 1 Hz rate. It sends estimates to the IMU strapdown navigator, which corrects the IMU specific force and angular rate measurements and then integrates them to obtain high frequency trajectory estimates. The velocity estimates are projected along each satellite's line of sight for tracking loop aiding. This configuration is called a tightly coupled system.
Standard GPS carrier and code tracking loops, with aiding from a medium grade IMU, normally break lock at jamming to signal ratios of 47 and 57 dB respectively. For high dynamic vehicles, these maximum jamming ratios can be achieved by aiding the tracking loops with IMU data as described above.
Recently, jamming to signal ratios above 60 dB have been attained by closing the tracking loops through the navigation filter. This multi-satellite tracking loop structure is derived from the maximum likelihood estimate of the pseudorange error given the composite GPS signal at the input to the tracking processor. In this configuration, position and velocity estimates from the strapdown navigator are projected along each satellite's line of sight to generate range and range rate estimates to each satellite. These drive numerically controlled oscillators whose outputs are mixed with the digitized I and Q data, processed, and input to the navigation filter as residual (measurement—estimated measurement) estimates for each satellite. The navigation filter outputs vehicle trajectory and IMU error estimates to the strapdown navigator to close the multi-satellite tracking loop.
Because every GPS satellite tracking loop includes the navigation filter, each satellite is integrated with the IMU and other tracked satellites. In this mode the satellites aid each other, and with the IMU, generate a more robust navigation solution.
While the systems described above can track through substantial jamming, the problem of acquisition in the presence of jamming remains.